Responsiveness of Brca1 and Trp53 Deficiency–Induced Mammary Preneoplasia to Selective Estrogen Modulators versus an Aromatase Inhibitor in Mus musculus

Abstract

An intervention study initiated at age 4 months compared the impact of tamoxifen (25mg), raloxifene (22.5mg) and letrozole (2.5mg) administered by 60-day-release-subcutaneous-pellet on mammary preneoplasia prevalence at age 6 months in conditional genetically engineered mouse models with different Brca1 gene dosages targeted to mammary epithelial cells and germline Trp53 haploinsufficiency (10-16/cohort). The proportion of unexposed control mice demonstrating mammary preneoplasia at age 6 months was highest in Brca1fl11/fl11/Cre/p53−/+ (54%) mice followed by Brca1WT/fl11/Cre/p53−/+ mice (30%). By age 12 months invasive mammary cancers appeared in 80% of Brca1fl11/fl11/Cre/p53−/+ and 42% of Brca1WT/fl11/Cre/p53−/+ control unexposed mice. The spectrum of cancer histology was similar in both models without somatic mutation of the non-genetically engineered Brca1, Trp53, Brca2 or Dapk3 alleles. Two months exposure to tamoxifen, raloxifene and letrozole significantly reduced estrogen-mediated tertiary branching by 65%, 71% and 78%, respectively, in Brca1fl11/fl11/Cre/p53−/+ mice at age 6 months. However, only letrozole significantly reduced HAN prevalence (by 52%) and number (by 30%) and invasive cancer appeared despite tamoxifen exposure. In contrast, tamoxifen significantly reduced HAN number by 95% in Brca1WT/fl11/Cre/p53−/+ mice. Control mice with varying combinations of the different genetically modified alleles and MMTV-Cre transgene demonstrated that the combination of Brca1 insufficiency and Trp53 haploinsufficiency was required for appearance of preneoplasia and no individual genetic alteration confounded the response to tamoxifen. In summary, although specific antihormonal approaches showed effectiveness, with Brca1 gene dosage implicated as a possible modifying variable, more effective chemopreventive approaches for Brca1-mutation-induced cancer may require alternative and/or additional agents.

Peroxisome proliferator-activated receptor gamma and BRCA1

“…BRCA1, DNA repair-associated gene (BRCA1), is one of the most highly associated cancer susceptibility genes, principally with breast and ovarian cancer (Turnbull et al 2018). Currently recommended preventive strategies are primarily surgical (Ludwig et al 2016, Andrews & Mutch 2017 with evaluation of the relative effectiveness of the lifestyle-driven (Lammert et al 2018) and hormonal approaches used, more generally, for reduction of breast cancer risk (Pujol et al 2012, Phillips et al 2013, Dabydeen et al 2015, Alothman et al 2017) considered more investigational.…”

“…Biological effects of known PPARG functions related to cell growth and differentiation could be stringently examined in the presence and absence of BRCA1. Measurements of downstream gene regulatory networks that follow PPARG activation could be evaluated and compared to determine if there were significant differences correlated with Brca1 gene dosage in murine cancer prevention models (Savic et al 2016, Alothman et al 2017.…”

Differential efatutazone's impact on mammary neoplasia dependent upon Brca1 dose

“…The study was prompted by a previous investigation demonstrating that efatutazone exposure beginning at age 4 months in Brca1 fl11/fl11/Cre/p53+/− mice, a model which typically generates a range of histology of triple-negative mammary cancers, significantly reduces mammary hyperplasia and induces more differentiated mammary cancers including some ER-positive adenocarcinomas (Nakles et al 2013). Questions raised by the earlier study included whether or not mice carrying a disruption in only one Brca1 allele (Alothman et al . 2017) might be more responsive to efatutazone and the possibility that earlier administration would be more effective in reducing mammary cancer development (Dong 2013).…”

“…An alternative or contributing hypothesis for the significant differences found would be a differential action of efatutazone based on the presence or absence of full-length Brca1. Brca1 WT/fl11/Cre/p53+/− mice still retain one functional Brca1 allele, and mammary cancer development occurs without loss of the second Brca1 allele (Alothman et al 2017), as occurs in a portion of women carrying BRCA1 mutation (Roy et al 2011). PPARG agonists are reported to increase BRCA1 expression in vitro in human breast cancer cells and in vivo in mammary glands of FVB mice (Subbaramaiah et al 2012).…”

“…All animal procedures were performed in accordance with federal guidelines and were approved by the Georgetown University Institutional Animal Care and Use Committee. Generation of mouse cohorts, dietary interventions (F3028, rodent diet, grain-based 1/2-in pellets without or with efatutazone (30mg/kg), Bio-Serv, Frenchtown, NJ, USA) and histology were performed as previously described (Nakles et al 2013, Alothman et al 2017). Pathology was read blindly by a board-certified pathologist (B V K).…”